1. Field of the Invention
The present invention generally concerns three-dimensional optical memory apparatus and memory media, and methods of using such apparatus and media. The present invention particularly concerns (i) three-dimensional volumes of active medium selectively both alterable and interrogatable by use of at least two intersecting beams of radiation, thereby to form a radiation memory; (ii) the manner of using the intersecting radiation beams and the physical and/or intersecting radiation beams and the physical and/or chemical effects of such use; (iii) the construction of binary-stated informational memory stores, three-dimensional patterns, and/or three-dimensional displays based on these effects; (iv) the manner of selectively directing intersecting radiation beams to intersect within three-dimensional volumes for purposes of addressing selected domains within such volumes, particularly as addressable memory stores; and (v) the manner of selectively impressing information on, or extracting information from, one or more intersecting beams of radiation in order that such information may be radiatively written to, or radiatively read from, a three-dimensional memory.
2. Background of the Invention
2.1 The General Requirement for Information Storage in Memories
The need for computerized data storage and processing has been increasing, in the past decade, at a high rate. In response to this need, semiconductor-based computer technology and architecture have greatly improved. However, barriers to further reducing the size and price of semiconductors may now be inhibiting development of even higher performance computers, and the more widespread use of high performance computers.
The major determinant of the size and price of high performance computers is the memory. The data storage requirements of new high performance computers, circa 1990, are very great, typically many gigabytes (10.sup.12 bits). New and improved, compact, low cost, very high capacity memory devices are needed. These memory devices should be able to store many, many gigabytes of information, and should randomly retrieve such information at the very fast random access speeds demanded by parallel computing.
An optical memory offers the possibility of packing binary-stated information into a storage medium at very high density, each binary bit occupying a space only about one light wavelength in diameter. When practical limitations are taken into account this leads to a total capacity of about 10.sup.11 bits for a reasonably-sized two-dimensional optical storage medium--the amount of information contained in about 3000 normal size books. A comparison of the optical memory to existing types of computer memories is contained in the following Table 1.
TABLE 1 ______________________________________ MEMORY ACCESS TYPE CAPACITY TIME COST ______________________________________ TAPE 10.sup.10 bits 100 sec 10.sup.-5 .cent./bit DISK 10.sup.8 bits 300 msec 5.times.10.sup.-2 .cent./bit DRUM 10.sup.7 -10.sup.8 bits 10 msec 10.sup.-2 .cent./bit CORE 10.sup.6 bits 1 .mu.sec 2.cent./bit SEMI- 10.sup.5 bits 100 nsec 20.cent./bit CONDUCTOR OPTICAL 10.sup.9 -10.sup.12 bits 10 nsec 10.sup.-3 -10.sup.-4 .cent./bit ______________________________________
The present invention will be seen to be embodied in a 3-D optical memory system. Any optical memory system, 3-D or otherwise, is based on light-induced changes in the optical, chemical and/or physical properties of materials.
2.2 Optical Recording Media, and the Use Thereof in Optical Memories
At the present two general classes of optical recording media exist, namely phase recording media and amplitude recording media. Recording on the first is based on light-induced changes of the index of refraction (i.e., phase holograms). Recording on the second media is based on photo-induced changes in the absorption coefficient (i.e., hole burning).
Volume information storage is a particularly attractive concept. In a two dimensional memory the theoretical storage density (proportional to 1/.lambda..sup.2) is 1.times.10.sup.11 bits/cm.sup.2 for=266 nm. However in a 3-D memory the theoretical storage density is 5.times.10.sup.16 bits/cm.sup.3. Thus the advantages of 3-D data storage versus previous two dimensional information storage media become apparent.
Volume information storage has previously been implemented by holographic recording in phase recording media. Reference F. S. Chen, J. T. LaMacchia and D. B. Fraser, Appl. Phys. Lett., 13, 223 (1968); T. K. Gaylord, Optical Spectra, 6, 25 (1972); and L. d'Auria, J. P. Huignard, C. Slezak and E. Spitz, Appl. Opt., 13, 808 (1974).
The present invention will be seen to implement volume writable-readable-erasable optical storage in an amplitude recording medium. One early patent dealing with three-dimensional amplitude-recording optical storage is U.S. Ser. No. 3,508,208 for an OPTICAL ORGANIC MEMORY DEVICE to Duguay and Rentzepis, said Rentzepis being the selfsame inventor of the present invention. Duguay and Rentzepis disclose an optical memory device including a two-photon fluorescent medium which has been solidified (e.g., frozen or dispersed in a stable matrix, normally a polymer). Information is written into a selected region of the medium when a pair of picosecond pulses are made to be both (i) temporally coincident and (ii) spatially overlapping within the selected region. The temporally-coincident spatially-overlapping pulses create, by process of two-photon absorption, organic free radicals which store the information at an energy level intermediate between a fluorescent energy level and a ground state energy level. The free radicals store the desired information for but a short time, and until they recombine. The information may be read out by interrogating the medium with a second pair of coincident and overlapping picosecond pulses. In the case where the medium is frozen solid, interrogation may also be accomplished by directing a collimated infrared light beam into the selected region, thereby causing that region to liquefy and permitting its contained free radicals to undergo recombination. In each of the aforementioned cases, the interrogation beam causes the interrogated region to selectively fluoresce in accordance with the presence, or absence, or free radicals. The emitted radiation is sensed by an appropriate light detector as an indication of the informational contents of the interrogated region.
This early optical memory of Duguay and Rentzepis recognizes only that two-photon absorption should be used to produce excited states (e.g., singlet, doublet or triplet states) of an active medium over the ground state of such medium. These excited states are metastable. For example, one preferred fluorescent medium is excitable from ground to a singlet state by process of two-photon absorption occurring in about 10.sup.-15 second. The excited medium will remain in the singlet state for about 10.sup.-8 second before fluorescing and assuming a metastable triplet state. This metastable state represents information storage. Alas, this metastable state will spontaneously decay to the ground state by fluorescence after about 1 second (depending on temperature). The memory is thus unstable to hold information for periods longer than about 1 second. It should be understood that the fluorescent medium of the Duguay and Rentzepis memory is at all times the identical molecular material, and simply assumes various excited energy states in response to irradiation.
Another previous optical system for accomplishing the volume storage of information, and for other purposes, is described in the related series of U.S. patent Ser. Nos. 4,078,229; 4,288,861; 4,333,165; 4,466,080; and 4,471,470 to Swainson, et al. and assigned to Formigraphic Engine Corporation. The Swainson, et al. patents are variously concerned with three-dimensional systems and media for optically producing three-dimensional inhomogeneity patterns. The optically-produced 3-D inhomogeneity patterns may exhibit (i) controlled refractive index distributions, (ii) complex patterns and shapes, or (iii) physio-chemical inhomogeneities for storing data. The Swainson, et al. patents generally show that some sort of chemical reaction between two or more reactive components may be radiatively induced at selected cell sites of a 3-D medium in order to produce a somewhat stable, changed, state at these selected sites.
U.S. patent Ser. No. 4,471,470, in particular, describes a METHOD AND MEDIA FOR ACCESSING DATA IN THREE-DIMENSIONS. Two intersecting beams of radiation are each matched to a selected optical property or properties of the active media. In one embodiment of the method and media, called by Swainson, et al. "Class I systems," two radiation beams generate an active region in the medium by simultaneous illumination. In order to do so, two different light-reactive chemical components are typically incorporated within the medium. Both components are radiation sensitive, but to different spectral regions. The two radiation beams intersecting in a selected region each produce, in parallel, an associated chemical product. When two products are simultaneously present in a selected intersection region then these products chemically react to form a desired sensible object. The sensible object may represent a binary bit of information. One or both of the radiation-induced chemical products desirably undergoes a rapid reverse reaction upon appropriate irradiation in order to avoid interference effects, and in order to permit the three-dimensional media to be repetitively stored.
In other embodiment of the Swainson et al. method and media, called "Class II systems," one of the radiation beams must act on a component of the medium before the medium will thereafter be responsive to the other radiation beam. The class I and class II systems thusly differ by being respectively responsive to the effects of simultaneously, and sequentially, induced photoreactions.
The Swainson, et al. patents--including those patents that are not directed to information storage and that are alternatively directed to making optical elements exhibiting inhomogeneity in their refractive index, or to making physical shapes and patterns--are directed to inducing changes in a bulk media by impingent directed beams of electromagnetic radiation, typically laser light, in order that selected sites within the bulk media may undergo a chemical reaction. There are a large number of photosensitive substances that are known to undergo changes in the presence of light radiation. The changed states of these substances are, in many cases, chemically reactive. The patents of Swainson, et al. describe a great number of these photosensitive and photoreactive substances. Such substances may generally be identified from a search of the literature.
Swainson, et al. also recognize that molecular excitation from a ground state to an excited state may occur following a stepwise absorption of two photons. Swainson, et al. call this "two-photon absorption." Swainson, et al. describe that a solution of 8' ally-6' nitro-1, 3, 3-trimethylspiro(2' N-1-benzopyran-2'-2-induline) in benzene may be exposed to intersecting synchronized pulsed ruby laser beams transmitted through an UV elimination filter to form, at the region of intersection, a spot of color. The process of stepwise absorption of two photons in this solution, and in others, is recognized by Swainson, et al , only as regards its use to produce an excited state that may form (as in the example) colored products, or that may serve as an energy transfer agent.
In making all manner of excited states--including singlet, doublet, triplet, and quartet states--the patents of Swainson, et al. describe known photochemistry. Generally chemistry, and photochemistry, that is known to work in one dimension is equally applicable in three dimensions. For example, it is known that an electron may be knocked off an active substance so that it becomes an ion. For example, it is known that radiation may cause a substance to dissociate a proton, again becoming an ion. For example, it is even known how to induce spin changes and changes in parity by electromagnetic radiation. Once these changes, or others, are induced in an active medium then Swainson, et al. describe a reliance on the transportive capabilities of the liquid or gaseous support media in order to permit a chemical reaction to transpire.
The present invention will be seen to reject the Swainson et al. approach of inducing chemical reactions in a 3-D medium by creating one or more reagents by use of radiation. One reason why the present invention does so is because the same support medium, or matrix, that offers those transportive capabilities that are absolutely necessary to permit the chemical reactions to occur will also permit, at least over time, undesired migration of reagents or reaction products in three dimensions, destroying the integrity of the inhomogeneity pattern.
2.3 Spatial Light Modulators
The three-dimensional optical memory of the present invention will be seen to employ spatial light modulators.
A recent survey, circa 1990, of spatial light modulators is contained in the article Two-Dimensional Spatial Light Modulators: A Tutorial by John A. Neff, Ravindra A. Athale, and Sing H. Lee, appearing in Proceedings of the IEEE Vol. 78, No. 5, May 1990 at page 826. The following summary is substantially derived from that article.
Two-dimensional Spatial Light Modulators (SLMs) are devices that can modulate the properties of an optical wavefront--such as the properties of amplitude, phase, or polarization--as a function of (i) two spatial dimensions and (ii) time in response to information-bearing control signals that are either optical or electrical. SLMs usefully form a critical part of optical information processing systems by serving as input transducers as well as performing several basic processing operations on optical wavefronts.
SLMs, although once considered simply as transducers that permitted the inputting of information to an optical processor, have a broad range of applications, and are capable of performing a number of useful operations on signals in the optical domain. Some of the more important functions that have been demonstrated with SLMs are: analog multiplication and addition, signal conversion (power amplification, wavelength, serial-to-parallel, incoherent-to-incoherent, electrical-to-optical), nonlinear operations, and short-term storage.
The functional capabilities of SLMs can be exploited in a wide variety of optical computering architectures. Applications of 1-D and 2-D SLMs encompass just about every optical signal processing/computing architecture conceived.
SLMs may be classified as to type. The major classification categories result from (i) the optical modulation mechanism, (ii) the variable of the optical beam that is modulated, (iii) the addressing mode (electrical or optical), (iv) the detection mechanism (for optically-addressed SLMs), and (v) the addressing mechanism (for electrically-addressed SLMs).
The modulation of at least one property of a readout light beam is inherent in the definition of an SLM. Hence the first major category of SLMs is based on modulation mechanisms. The modulation mechanism employs an intermediate representation of information within a modulating material. An information-bearing signal, either optical or electrical, is converted into this intermediate form. The major forms of conversion mechanisms that are employed in 2-D SLMs are
(a) Mechanical
(b) Magnetic
(c) Electrical
(d) Thermal.
Of these conversion mechanisms, the electrical mechanism will be seen to be preferred for use in the three-dimensional optical memory of the present invention. In the electrical conversion mechanism, the electric field interacts with the modulating material at several levels, giving rise to different effects. The interaction can take the form of distorting the crystal lattice, changing the molecular orientations, or modulating the electron density functions.
A conversion mechanism and the modulating material so converted have a characteristic response time, activation energy, and spatial scale. These parameters, in turn, have a major impact on the respective speed sensitivity and spatial resolution of the optical modulation performed by the SLM. A modulation mechanism, however, becomes physically more specific only when combined with a choice of appropriate modulation variables, to be discussed next.
An optical wavefront has several associated variables that can be modulated as a function of the spatial coordinates and time in order to carry information. These variables include
(a) Intensity (amplitude)
(b) Phase
(c) Polarization
(d) Spatial frequency spectrum (texture).
Intensity (amplitude) and phase are the most commonly used representations in an optical computing system. Polarization and spatial frequency spectrum are often used as intermediate representations, and are converted into intensity or phase modulation before the information is used in the next stage of the optical computing system. Intensity (amplitude), phase, and polarization modulation will each be seen to be employed in the three-dimensional optical memory of the present invention.
Intensity, or amplitude, modulation commonly results when the absorption characteristics of a modulating material are changed. Because the intensity of a light beam is proportional to the square of its amplitude, the difference between these two modes depends on the variable that is employed in subsequent processing of a SLM output. The present invention will be seen to be more concerned with selectively controllably spatially modulating to zero intensity, and amplitude, then with any requirement that modulation at and to an opposite binary state should produce sufficient intensity, and amplitude, so as to permit a desired operation within an optical memory. This is because any presence of light intensity, or amplitude, in those spatial locations of an optical wavefront (i.e., at a particular time) where, and when, there is desirably no light intensity, nor any amplitude, constitutes optical noise.
The three-dimensional optical memory in accordance with the present invention will be seen to be innately highly insensitive to optical noise, being roughly sensitive to (noise/signal).sup.2, as opposed to the lesser figure of merit noise/signal, in certain operations. Nonetheless to this innate insensitivity, optical noise may be cumulative in degrading the integrity of informational stores within the optical memory over billions and trillions of read and write cycles. Accordingly, intensity, or amplitude, modulation in accordance with the present invention is desirably very "clean," with minimal, essentially zero, optical intensity or amplitude in those wavefront regions which are spatially modulated to one ("0") binary state. Spatial light modulation, and SLMs, will be seen to so operate in the present invention: veritably no light will be in regions where it is not wanted.
Polarization modulation is commonly achieved by modulating the birefringence associated with the modulating material of the SLM. Birefringence is a property of some materials in which the refractive index depends on the state of polarization and direction of light propagation. Depending upon the effect utilized, the state of polarization changes (e.g., from linear to elliptical), or the angle of the linear polarization changes without changing the state of polarization. The memory system of the present invention will be seen to use phase-modulating SLMs that produce each such effect.
Polarization modulation can be changed into intensity (amplitude) modulation by employing polarized readout light and an analyzer in the output. The memory system of the present invention will later be seen to be so change polarization modulation into intensity modulation. Indeed, this will be seen to be a primary approach by which the net effective intensity, or amplitude, modulation will be rendered exceptionally "clean," and of satisfactory quality to support reliable operation of the three-dimensional optical memory over great periods of time and astronomical numbers of read and write cycles.
2.4 The Figure of Merit of a Readable and Writable and Erasable Optical Memory
Most new memory technologies are typically immediately gauged by the figures of merit that have attended past technologies. These previous figures of merit, while generally representing criteria that must be met by an operational memory, are often substantially irrelevant to the truly critical performance aspects, and new figures of merit, appropriate to a new technology.
For example, the Intellectual Property Owners, Inc. gives annual awards in the name of its educational subsidiary the IPO Foundation to distinguished inventors. In the 1989 awards, Robert P. Freese, Richard N. Gardner, Leslie H. Johnson and Thomas A. Rinehart were honored for their improvements in erasable, rewritable optical disks introduced by the 3M Company during 1988. The optical disks can store 1,000 times as much information as conventional flexible diskettes used with personal computers. The inventors were the first to achieve a signal to noise ratio for an erasable optical disk in excess of 50 decibels.
Although the inventors of the present invention would be the first to recognize this contribution, and to acknowledge the necessity of an adequate optical (and electrical) signal-to-noise ratio for optical memories, a focus on signal-to-noise as a figure of merit may be rooted in the importance of this measurement in certain previous electrotechnology. For example, certain magnetic memories, such as garnet film and Block line memories, have undesirably small signal-to-noise ratios.
It is uncertain what constitutes the ultimate, or even the most appropriate, figure of merit (or figures of merit) for a readable and writable and erasable optical memory. However, it is suggested that, in the case of a three-dimensional optical memory, it is important to consider whether or not, and how fast, the memory might become "dirty" from use and suffer degradation in the integrity of its data stores.
The concept of a "dirty" three-dimensional optical memory arises because every read and write operation on the memory by use of radiation has the potential to perturb other storage domains than just those domains that are intended to be dealt with. The most analogous prior memory technology may be the original square loop ferrite magnetic core memories. In these early core memories many millions of interrogations of one memory location may cause a single magnetized core having a weak hysteresis to fail to provide a sense signal adequate for detection of its magnetic condition, meaning the binary data bit stored. Even more relevantly, unaddressed and/or unwritten cores, commonly in physically proximate positions, may sometimes inadvertently and erroneously change hysteresis state, causing attendant loss of data.
A three-dimensional optical memory is analogous. The radiation that is used to read and write selective domains of the memory can, if great care is not employed, end up, after millions or billions of cumulative cycles, changing domains other than those domains that are desired to be changed. Such an undesired change of domains degrades the integrity of the data stored within the memory.
Accordingly, the present invention concerns not only addressably reading and writing and erasing a three-dimensional optical memory and doing so at impressive levels of performance, but doing so by design, at a high figure of merit. A "high figure of merit" means that an optical memory constructed in accordance with the invention is practically and reliably useful in the real world, reliably storing and reading any and all data patterns with absolute integrity during indefinitely long periods of any pattern of use, or non-use, whatsoever. Consider that three-dimensional optical memories, storing information in a volume that is little more than a cube of plastic, are intrinsically physically amorphous and homogenous. It is prudent to use some care, and forethought, in the manner of radiative reading and writing of such a volume so that those changes that are selectively induced within selected domains of the volume should be absolutely stable and independent. Nothing should be done, or repetitively done, on any selected domains that adversely affects the integrity of non-selected domains.